The present invention relates to electronic ballasts for powering a discharge lamp, and a lamp fixture including various embodiments of such a ballast.
Electronic ballasts for powering a discharge lamp of a thermionic cathode type such as a high pressure discharge lamp, which is also called an HID (high-intensity discharge) lamp, are conventionally known in the art and may generally include a DC-AC power converter circuit and a control circuit for controlling the power converter.
Certain control circuits as known for a discharge lamp ballast of this type may perform a starting operation to ignite the discharge lamp by making the voltage output from the power converter relatively high, a filament heating operation to make the frequency of power output from the power converter relatively high for heating each filament of the discharge lamp, and a normal operation to cause the power converter to maintain an output AC power for stable lighting of the lamp.
Accordingly, lamp operation is stabilized after transition to a normal operation, in comparison with a case where filament heating is not performed and discharge is suppressed which may undesirably extinguish the lamp.
Referring to FIG. 26(a), a starting period P1 for performing a starting operation is followed by a filament heating period P2 for performing a filament heating operation, and a normal period P3 for performing normal lamp operation. If the filament heating period P2 is relatively short, the lamp filament is not heated sufficiently before the start of the normal period P3, wherein current output to the discharge lamp (referred to as a “lamp current” hereinafter) becomes asynchronous in polarity. Transition to normal operation without sufficient heating of the lamp filament will result in unstable discharge after transition to normal operation and the lamp may even be extinguished. Accordingly, it is necessary to make the filament heating period P2 sufficiently long as shown in FIG. 26(b), but a required length of the filament heating period P2 (i.e., a duration of filament heating operation) varies with respect to each lamp.
Accordingly, if the duration of a filament heating is determined with reference to a discharge lamp which requires a relatively long period of time to perform a filament heating operation with respect to all the lamps assumed to be connected, this duration is likely excessive to the other lamps. Because the filament heating operation causes a power converter to output power of greater amplitude than in the normal operation, the duration of the filament heating needs to be as short as possible in order to prevent adverse effects to discharge lamp life.